This application claims the priority of Japanese Patent Application No. 2000-002626 filed on Jan. 11, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical system for an interferometer which measures the planarity of a sample surface by utilizing interference fringes formed by interfering actions of light; and, more specifically, to an optical system employed in an oblique incidence interferometer which can measure the planarity of a rough surface, in particular, in a noncontact fashion by obliquely irradiating a reference plane and a sample surface with light, and an apparatus using the same.
2. Description of the Prior Art
Various interferometer apparatus have conventionally been known for measuring the planarity of surfaces of processed products. Among them, oblique incidence interferometer apparatus have been known as an apparatus which can measure the planarity of sample surfaces having a large difference in surface level.
Since the measurement sensitivity can be set low by making a coherent luminous flux obliquely incident on a sample surface, the oblique incidence interferometer apparatus have been in use for measuring the planarity of rough surfaces and the like in a noncontact fashion. Here, letting xcex be the wavelength of light used for measurement, and xcex8 be the incident angle of light incident on the entrance surface of a reference standard, the amount of unevenness of the sample surface, i.e., the measurement sensitivity xcex94h, is represented by the following expression:
xcex94h=xcex/(2cos xcex8)
Namely, as the incident angle xcex8 increases, so that the degree of oblique incidence is greater, the fringe interval increases, so that the measurement sensitivity can be lowered, whereby surfaces with a lower planar precision can be measured.
FIG. 5 shows a conventional configurational example using a planar reference plate as a reference standard. This oblique incidence interferometer apparatus is configured such that a reference plane 116a of a plane-parallel plate 116 and a sample surface 2a of a sample 2 are disposed so as to face each other, the reference plane 116a is obliquely irradiated with coherent light which is turned into parallel rays by a collimator lens 114 after being emitted from a laser light source 111, and interference fringes corresponding to the optical path difference based on the distance between the reference plane 116a and the sample surface 2a are projected onto a screen 118, so as to be viewed by an observer 119. The principle of generating interference fringes is shown in FIG. 7A. Namely, of a parallel luminous flux incident on a reference plane 6a from the upper left side of the paper surface, a part is emitted from the reference plane 6a so as to be made obliquely incident on the sample surface 2a, and then is reflected by the sample surface 2a so as to be made incident on the reference plane 6a again, thereby optically interfering with the luminous flux internally reflected by the reference plane 6a, thus generating interference fringes.
In this configuration, however, interference noise such as that shown in FIG. 7B is likely to occur. This interference noise is caused by light reflected by a surface (which is the entrance surface and exit surface) of the planar plate other than the reference plane, and is eliminated to a certain extent if this surface is provided with an antireflection coating. Since the incidence angle is large, however, a coating having a low reflectance is hard to apply.
FIG. 6 shows a conventional configurational example known as Abramson type using a rectangular equilateral triangle prism as a reference standard. In FIG. 6, members similar to those in the oblique incidence interferometer shown in FIG. 5 are referred to with numerals having two lower digits identical to those of their corresponding members in FIG. 5. This apparatus is configured such that interference fringes projected onto a screen 218 are captured by a TV camera 219, so as to be viewed. This Abramson type apparatus can prevent interference noise from occurring due to a surface reflection such as that shown in FIG. 7B.
However, a problem of interference noise caused by multiple reflection light between the sample surface and reference plane remains as shown in FIG. 7C. For eliminating this problem, the apparatus must be configured such that light reflected in a multiple fashion between the sample surface and reference plane is prevented from interfering with the light internally reflected by the reference plane and thereby reaching the screen surface. An example of techniques effective in eliminating this problem with a relative arrangement of optical members is one in which the reference plane is set to a size which is at least twice as large as that of the sample surface. This conventional configurational example using a rectangular equilateral triangle prism is problematic in that the prism itself becomes very large and heavy in order to make the reference plane greater.
While the luminous flux directed from the collimator lens to the sample surface must cover the parts of incident light onto the sample surface and reference light, a parallel luminous flux having a large luminous flux diameter is needed to be made incident in this conventional configurational example using the rectangular equilateral triangle prism, since the incident angle with respect to the entrance surface of prism is small. It is problematic in that the collimator lens also becomes greater, whereby the interferometer as a whole enhances its size.
Further, there is a problem that the luminous flux internally reflected by the reference plane of prism is emitted in the same direction as interference light, so that the noise based on this internally reflected light is superimposed on the screen.
In view of such circumstances, it is an object of the present invention to provide an oblique incidence interferometer optical system which can make the optical system smaller and can reduce the weight and size of an interferometer apparatus while lowering interference noise in an oblique incidence interferometer yielding a large incident angle with respect to a sample surface and a moderate fringe sensitivity; and an oblique incidence interferometer apparatus using the same.
The present invention provides an oblique incidence interferometer optical system comprising:
a light source for emitting coherent light;
a collimator lens for turning the light into a parallel luminous flux;
a reference standard having an entrance surface on which the luminous flux is incident and a reference plane on which the luminous flux entering from the entrance surface is obliquely incident; and
a screen section on which an interference fringe is projected, the interference fringe being formed by a part the luminous flux reflected by a sample surface disposed so as to oppose the reference plane after being emitted from the reference plane and a part of the luminous flux internally reflected by the reference plane;
wherein the entrance surface and reference plane of the reference standard form therebetween an angle set within the range of 10xc2x0 to 30xc2x0.
Preferably, the reference standard comprises an exit surface provided with the screen section.
Preferably, the reference plane and the screen section are formed parallel to each other.
Preferably, the reference plane comprises a prism.
The apparatus in accordance with the present invention comprises the oblique incidence interferometer optical system.
Preferably, the apparatus is configured such that the interference fringe projected on the screen section is viewed with a camera disposed face to face with the exit surface.
Preferably, the apparatus further comprises optical path changing means for changing the incident angle of the luminous flux incident on the entrance surface.